If cost is the issue, there are some clues to make a choice: fusion is expensive equipment and cheap splices, while mechanical is cheap equipment and expensive splices. So if
you make a lot of splices (like thousands in an big telco or CATV network) use fusion splices. If you need just a few, use mechanical splices.
Fusion splices give very low back reflections and are preferred for singlemode high speed digital or CATV networks. However, they don’t work too well on multimode splices, so
mechanical splices are preferred for MM, unless it is an underwater or aerial application, where the greater reliability of the fusion splice is preferred.